We measured erythrocyte, plasma, osteocyte, extravascular extracellular, and total water fluid spaces and calculated their K content in cortical bone of dogs of various age. Total and exchangeable K were measured by atomic absorption spectroscopy and by volume of distribution techniques, respectively. All fluid spaces in bone decreased with increasing age of the dogs. The total and exchangeable K contents of cortical bone and the K content calculated from the fluid spaces were within the same range in each age group. These values all fell with increasing age.